Decouple test cases from system under test

production.py

import pytest


class ClassA(object):
    def apply(self, *args):
        return 42


class ClassB(object):
    def apply(self, *args):
        return 0


def func_gamma(something):
    ...


def func_delta():
    ...

test_classes.py

@pytest.mark.parametrize(argnames='correct_class', argvalues=[ClassA, ClassB])
def test_decoupled_from_class(correct_class):
    # arrange
    params = "hello"

    # act
    object_under_test = correct_class()
    actual = object_under_test.apply(params)

    # assert
    expected = 42
    assert actual == expected

test_functions.py

@pytest.mark.parametrize(argnames='function_under_test', argvalues=[func_delta, func_gamma])
def test_decoupled_from_function(function_under_test):
    # arrange
    params = "hello"

    # act
    actual = function_under_test(params)

    # assert
    expected = 42
    assert actual == expected